1. Field of the Invention
The present invention relates to a radiation detector and a radiological image radiographing apparatus. In particular, the present invention relates to a radiation detector which detects an emitted radiation and a radiological image radiographing apparatus which radiographs a radiological image expressed by the radiation detected by the radiation detector.
2. Description of the Related Art
In recent years, a radiation detector such as an FPD (Flat Panel Detector), which has a radiation-sensitive layer disposed on a TFT (Thin Film Transistor) active matrix substrate which can convert a radiation such as an X-ray directly into digital data, has been put to practical use. A radiological image radiographing apparatus using this radiation detector is advantageous in that an image can be immediately checked and accordingly fluoroscopy (moving image radiographing), which is for radiographing a radiological image continuously, can be performed compared with a radiological image radiographing apparatus using an X-ray film or an imaging plate in the related art.
As such a radiation detector, various types of radiation detectors have been proposed. For example, there is an indirect conversion type radiation detector in which a radiation is first converted into light by a scintillator, such as CsI:Tl or GOS (Gd2O2S:Tb), and the converted light is converted into electric charges and stored in a sensor section, such as a photodiode. In the radiological image radiographing apparatus, the electric charges stored in the radiation detector are read as an electric signal, and the read electric signal is amplified by an amplifier and is then converted into digital data by an A/D (analog to digital) converter.
Meanwhile, there has been a radiation detector with a phosphor layer (scintillator), which includes columnar crystals with relatively high sensitivity, in order to reduce the amount of exposure to a subject (patient).
In this technique, in order to increase the amount of radiation absorbed by the columnar crystals, it is necessary to make a scintillator layer considerably thick, as is also apparent from FIG. 11 in JP2008-51793A as an example. However, an increase in the thickness of the scintillator layer leads to an increase in cost. In addition, as the thickness increases, it is necessary to increase the porosity in an initial portion (base portion) of the columnar crystals. As a result, there has been a problem in that the amount of emitted light in the initial portion is reduced.
That is, the diameter of a columnar portion changes with a predetermined fluctuation during the vapor deposition of the columnar crystals. Therefore, as the thickness increases, a probability that the maximum value of the fluctuation will occur is increased. As a result, a possibility that columnar portions will contact each other is increased. In addition, once columnar portions contact each other, a possibility that the columnar portions will be fused is increased. This leads to blurring of an image. In addition, there is also a predetermined fluctuation in the length of the columnar portion. Accordingly, if there is adhesion of foreign matter on the substrate on which the columnar portions are vapor-deposited, the length of an abnormally grown columnar portion also increases as the thickness increases. For this reason, a process of reducing the length of an abnormally grown columnar portion by pressure or the like is required after the vapor deposition process. This makes the manufacturing process complicated. In addition, a normal columnar portion around the abnormally grown columnar portion may be damaged due to the pressure. For this reason, when the scintillator layer is made thick in order to prevent the above-described fusion, it is necessary to set the filling rate of columnar crystals low (set the porosity of the initial portion high) in advance in order to prevent the above-described fusion and to prevent the complication of the process due to abnormal growth of columnar portions and damage to normally grown columnar portions. For example, WO2010/007807A discloses a scintillator in which the filling rate of columnar crystals is set to 75% to 90% when the thickness of the scintillator layer of the columnar crystals is 100 μm to 500 μm or more. In addition, JP2006-58099A discloses a scintillator in which the filling rate of columnar crystals is set to 70% to 85% when the thickness of the scintillator layer of the columnar crystals is 500 μm or more.
As a technique which can be applied to solve the above-described problems, JP2002-181941A discloses a radiological digital image radiographing apparatus that is excellent in sharpness and has high detection efficiency. Specifically, JP2002-181941A discloses a radiological digital image radiographing apparatus which has a phosphor layer formed of phosphor particles and binder resin and is characterized in that the phosphor layer is configured to include a first phosphor layer with a plate shape and a second phosphor layer which is provided in contact with the first phosphor layer and provided corresponding to each pixel and which has an approximately columnar shape.
In addition, JP2002-181941A discloses a configuration in which the approximately columnar second phosphor layer, the plate-shaped first phosphor layer, and a substrate where a photoelectric conversion element is provided are laminated sequentially from the emission side of radiation.
Moreover, in order to provide a radiological image detector capable of improving the light conversion efficiency and acquiring a high-quality image, JP2010-121997A discloses a radiological image detector in which a wavelength conversion layer including a phosphor, which receives a radiation and converts the radiation into light with a longer wavelength than the radiation, and a detector, which detects the light converted by the wavelength conversion layer and converts the light into an image signal showing a radiological image, are laminated and which is characterized in that the wavelength conversion layer is formed by laminating at least two layers of a first phosphor layer and a second phosphor layer, the second phosphor layer and the first phosphor layer are disposed in this order from the detector side, and the first phosphor layer includes absorbent to absorb the light converted by the first phosphor layer.
In addition, JP2010-121997A discloses a configuration in which a substrate where a photoelectric conversion element is provided, a plate-shaped second phosphor layer formed of GOS, and a columnar first phosphor layer formed of CsI are laminated sequentially from the emission side of radiation.